The present invention relates to two-part structural adhesives useful for bonding metal and polymeric substrates and more particularly to acrylate and methacrylate-based two-part structural adhesives and coatings.
Structural adhesives have found broad use in the modern-day manufacturing environment. In the automobile industry, for example, the adhesive bonding of metals to plastics has found wide application where the use of rivets or other mechanical fasteners is impractical due to considerations of aesthetics, cost or corrosion (due to the use of dissimilar metals). Such adhesive bonding applications have found broader acceptance due to recent advancements in formulations of adhesives, which allow effective bonding to metals (see U.S. Pat. No. 4,293,665 and U.S. Pat. No. 4,452,944, both assigned to Lord Corp.), bonding to solvent-resistant plastics (see U.S. Pat. No. 4,714,730, assigned to ITW, Inc.) and for low temperature applications (see U.S. Pat. No. 4,942,201, assigned to ITW, Inc.). The teachings of these four references are hereby incorporated by reference.
The effectiveness and strength of these adhesives allows for the substitution of adhesives for mechanical connectors such as bolts and rivets. In metal-bonding applications, such mechanical connectors can have disadvantages, such as appearance, cost or corrosion due to the dielectric effects of dissimilar metals. Many manufacturing situations employ adhesives to effect the bonding of two substrates, such as plastic to metal and metal to metal for reasons of economy and aesthetics.
Acrylate and methacrylate adhesives are generally known in the industry. U.S. Pat. No. 4,536,546 assigned to ITW of Glenview, Ill., the contents of which are incorporated herein by reference, describes a methacrylate based adhesive which employs a methacrylate ester monomer with a aldehyde-amine catalyst, a chlorinated polyethylene polymer and a graft copolymer. U.S. Pat. Nos. 4,942,201 and 4,714,730, also assigned to ITW, the contents of which are incorporated herein by reference, describe adhesives made with methacrylate and acrylate based adhesives together with various other additives and ingredients for use in the construction of large objects.
Metal bonding adhesives are also described in the art. U.S. Pat. No. 4,452,944, assigned to Lord Corp., describes a formulation composed of acrylates and methacrylates monomers, polymerizable non-acrylic monomers (styrenes), isocyanate functional prepolymers reacted with polyols, phosphorous compounds, oxidation/reduction systems capable of effecting a room temperature cure, tertiary amines as accelerators, metal molybdates and polybasic lead salts of phosphoric acid. The use of urethane prepolymers and the absence of carboxylic acids in this reference is intended to improve resistance to heat degradation of the adhesive.
In view of the above, it is apparent that progress in the development of metal bonding adhesives is an ongoing process. However, there is still a continuing need for improved adhesive systems. Therefore, it is an object of the present invention to provide an improved two-part adhesive system.
The present invention provides two-part adhesive systems that exhibit improved onset of handling strength as well improved-open times. In a first embodiment, a two-part system is provided which includes an adhesive part and an activator part. The adhesive part is a mixture formed from (I) an ethylenically unsaturated monomer, (II) a metal molybdate, (III) a metal salt of an ethylenically unsaturated carboxylic acid in an amount less than 3 weight percent, and (IV) an ethylenically unsaturated carboxylic acid. The activator part includes a free radical generator. Preferred ethylenically unsaturated monomers include acrylate or methacrylate ester monomers with methyl methacrylate being particularly preferred. The adhesive part can further include an admixed elastomeric material having a Tg less than xe2x88x9225xc2x0 C. and soluble in the ethylenically unsaturated monomer. Preferred elastomeric materials are polychloroprene, polyacrylonitrile-butadiene copolymers, copolymers of styrene and isoprene, copolymers of styrene and butadiene, carboxylated polychloroprenes, carboxylated polyacrylonitrile-butadiene copolymers, copolymers of ethylene and vinyl acetate, copolymers of styrene and olefinically unsaturated hydrocarbons, polybutylene, acrylate-based elastomers and mixtures thereof. One particularly preferred elastomer is polychloroprene.
The adhesive part can further include additional components such an admixed phosphorous-based adhesion promoter or an admixed core-shell impact modifier swellable in the ethylenically unsaturated monomer. A preferred phosphorous-based adhesion promoter is an acrylate or methacrylate ester phosphate. Preferred impact modifiers are MBS impact modifiers, ASA impact modifiers, ABS impact modifiers and mixtures thereof, with MBS being particularly preferred. The adhesive part can further include at least one admixed reducing agent.
The metal molybdates of the adhesive part preferably contain divalent metals with zinc being particularly preferred. Likewise, the metal cations for metal salt of the ethylenically unsaturated carboxylic acid are preferably divalent metals. Preferred unsaturated carboxylic acids for the adhesive part are acrylic and methacrylic acid. One particularly preferred metal salt of acrylic or methacrylic acid is zinc dimethacrylate. More preferred amounts of the metal salt of the ethylenically unsaturated carboxylic acid range from about 0.25 to about 2.5 weight percent, with about run 0.5 to about 2 weight percent being more preferable.
The ratio of the adhesive part to the activator part is preferably from about 20:1 to about 1:1, with about 15:1 to about 4:1 being more preferable. Free radical initiators for the activator part are preferably peroxides, hydroperoxides, and mixtures thereof, with benzoyl peroxide being particularly preferred. The activator part can further include an admixed epoxy resin such as a diglycidyl ether of Bisphenol-A. The activator part can also further include an admixed plasticizer.
In another embodiment, the present invention provides a curable adhesive composition including a mixture of an adhesive part and an activator part. The adhesive part includes (I) an ethylenically unsaturated monomer, (II) a metal molybdate, (III) a metal salt of an ethylenically unsaturated carboxylic acid in an amount less than 3 weight percent, and (IV) an ethylenically unsaturated carboxylic acid. The activator part includes a free radical generator. Preferred embodiments are as previously described.
In another embodiment, the present invention provides a laminate including a first substrate and a second substrate bonded thereto with a cured adhesive composition. The adhesive composition includes, in an uncured state, a mixture of an adhesive part and an activator part. The adhesive part includes (I) an ethylenically unsaturated monomer, (II) a metal molybdate, (III) a metal salt of an ethylenically unsaturated carboxylic acid in an amount less than 3 weight percent, and (IV) an ethylenically unsaturated carboxylic acid. The activator part includes a free radical generator. Preferred embodiments are as previously described.
The present invention also provides a method of preparing a laminate which includes contacting the surface of a first substrate with the surface of a second substrate with an adhesive composition therebetween. The adhesive composition includes a mixture of an adhesive part and an activator part, where the adhesive part includes (I), an ethylenically unsaturated monomer, (II) a metal molybdate, (III) a metal salt of an ethylenically unsaturated carboxylic, acid in an amount less than 3 weight percent, and (IV) an ethylenically unsaturated carboxylic acid, and where the activator part includes a free radical generator. Preferred embodiments are as previously described. Substrates to be bonded are plastics, metals and combinations thereof. In an additional preferred embodiment, the surfaces of the substrates omit a primer prior to contacting.
In an alternative embodiment, the present invention provides a two-part adhesive system which includes an adhesive part being a mixture formed from (I) an ethylenically unsaturated monomer, (II) a metal molybdate, (III) an ethylenically unsaturated carboxylic acid, where the adhesive part is substantially free of a metal salt of an ethylenically unsaturated carboxylic acid, and an activator part which includes a free radical generator. Curable adhesive compositions, laminates, methods of laminating using this alternative two-part adhesive system are also provided.
Advantageously, the adhesives of the present invention provide an improved onset of handling strength while at the same time providing superior bond strength and fatigue resistance. These and other advantages of the present invention will become more apparent from the description set forth below.